Final Report: Robust, Tunable Diode Lasers for Environmental MonitoringEPA Contract Number: EPD04063
Title: Robust, Tunable Diode Lasers for Environmental Monitoring
Investigators: Anderson, Michael H.
Small Business: Vescent Photonics, Inc.
EPA Contact: Manager, SBIR Program
Project Period: April 1, 2004 through June 30, 2005
Project Amount: $225,000
RFA: Small Business Innovation Research (SBIR) - Phase II (2004) Recipients Lists
Research Category: Small Business Innovation Research (SBIR) , Ecological Indicators/Assessment/Restoration , SBIR - Monitoring
The objective of this Phase II research was to develop a new and novel tunable diode laser that can be an alternative to distributed feedback (DFB) lasers or mechanical external cavity diode lasers (ECDL). DFB lasers are easily tuned, robust, and represent the industry standard for laser diode spectrometers for trace-gas sensing. They have a small tuning range, however, that limits their utility for some applications. For example, a laser that can tune over the entire R and P branches of a molecular transition can measure the temperature of the gas as well as concentration.
In Phase II Vescent Photonics, Inc., developed a novel planar-waveguide external cavity that can be used to tune low-cost laser diodes. A liquid crystal material was placed over the planar waveguide surface to enable voltage control of the guided light index via simple patterned electrodes similar to how a common liquid crystal display is manufactured. Vescent Photonics designed and tested a waveguide external cavity semiconductor laser (WECSL®) for operation in the 1820 nm range where inter-combination transitions of the NO molecule exist. Vescent Photonics also built and tested devices that operated at 1340 nm as an intermediate step and demonstrated the wide wavelength range of the technology. As part of the development effort, Vescent Photonics designed robust antireflection coatings for the laser diodes, examined suitable laser diodes, and developed coupling optics, adiabatic tapers, and packaging methods.
The Phase II research demonstrated a WECSL® that could tune 35 nm about a central wavelength of 1340 nm in a small centimeter-sized package, resulting in a great reduction in complexity and cost as compared to mechanically tuned lasers and a much larger electronic tuning range than possible with DFB lasers (see Figure 1). This laser could reach water transitions in the 1360 nm range and can easily be extended to 1400 nm where the major water lines reside, by choice of a suitable laser. The laser had 30 dB of side mode suppression and emitted over 10 mW of light. Vescent Photonics also demonstrated a 15 nm of tuning about a central wavelength of 1783 nm, largely covering the P branch of NO with several mW of light output. The tuning range largely agreed with models that predict 100 nm tuning ranges for optimized liquid crystal waveguide designs (see Figure 2).
Figure 1. Tuning Range of a 1340 nm WECSL®
While significant work remains to be completed before WECSL® lasers are ready for commercialization, this research has shown that the concept of a liquid-crystal waveguide can be effectively used to tune a laser diode over wide ranges with entirely electronic control. Furthermore, the technology has been demonstrated using commonly available laser diodes over a broad wavelength range. The laser hardware prior to assembly is depicted in Figure 3.
The primary commercial applications for these lasers will be for applications where a long electronic tuning range is needed in a low-cost, mechanically robust package. A longer tuning range can be used to sample multiple lines in a series of ro-vibrational molecular transitions or multiple trace-gas species. In combustion furnaces this laser can be used to measure the temperature of gas species as well as their concentration providing information to increase the efficiency and performance of the combustor. WECSL® can be used to probe NOX species, Methane, Ammonia, CO, CO2 and many other trace gas species that are important to agriculture, transportation, and energy production.
Figure 2. Tuning Data for a 1.8 μm WECSL®
Figure 3. Laser Hardware Prior to Assembly (Photo is for a 1.4 μm Laser).